Plant Cell

Structures

Cell Structure

In 1655, the English scientist Robert Hooke coined the term “cellulae” for the small box-like structures he saw while examining a thin slice of cork under a microscope.

TWO TYPES OF CELLS

Prokaryotes

•single compartment and surrounded

by the plasma membrane

•lacks a defined nucleus

•simple internal organization

•ex. bacteria, blue-green algae

(cyanobacteria)

Eukaryotes

••contain a defined membrane-bound nucleus

• extensive internal membranes that enclose

compartments

main parts: plasma membrane, cytoplasm and nucleus

• ex. all members of the plant and animal kingdom, fungi

(molds* and yeasts**), protozoan*

Prokaryotes Eukaryotes

Cell membrane

Ribosomes

Cell wall

Nucleus

Endoplasmic reticulum

Golgi apparatus

Lysosomes

Vacuoles

Mitochondria

Cytoskeleton

Animal Cells Plant Cells

Lysosomes

Cell membrane

Ribosomes

Nucleus

Endoplasmic reticulum

Golgi apparatus

Vacuoles

Mitochondria

Cytoskeleton

Cell Wall

Chloroplasts

The Structure of Eukaryotic Plant Cell

• A living plant cell is composed of three main parts namely: cell wall, protoplast and inclusions

1. The Cell

Wall

• The outermost part of the plant cell

• Non-living structure with main chemical

component cellullose (a polysaccharide)

• Other substances which may form part of the

cell wall are lignin, suberin and cutin

• Between neighboring cell walls is a

cementing intercellular layer composed of

pectin substance

• The nonliving layer is often referred to as the

middle lamella

PROTOPLASM

a mass of proteins, lipids, nucleic acids,

and water within a cell; except for the

wall, everything in the

cell is protoplasm

• Portions of the protoplast is organized into

protoplasmic bodies with specific functions and

are generally referred to as ORGANELLES

• The protoplast is composed of two main regions,

namely: an outer region called as the

CYTOPLASM and an inner region as the

NUCLEUS

• Is bounded by a protoplasmic membrane

called the plasma membrane,

plasmalemma or cell membrane

• The plasma membrane is a selectively

permeable protoplasmic membrane which

regulates the entry and exit of materials in

a cell

Other main cytoplasmic structures of plant cells:

1. Mitochondria

2. Ribosomes

3. Endoplasmic Reticulum (ER)

4. Golgi bodies or dictyosomes

5. Lysosomes

6. Plastids

7. Microtubules

Mitochondria

• Mitochondria are found in PLANT and animal cells.

• Sites of cellular respiration, ATP synthesis

• Bound by a double membrane surrounding fluid-filled matrix.

• The inner membranes of mitochondria are cristae

• The matrix contains enzymes that break down carbohydrates and the cristae house protein complexes that produce ATP

THE POWER

HOUSE OF THE

CELLATP

Ribosomes• Granular structures visible under electron

microscope

• Protein synthesis

• Ribosomes are composed of a large subunit

and a small subunit.

• Ribosomes can be found alone in the

cytoplasm, in groups called polyribosomes, or

attached to the endoplasmic reticulum.

• Ribosomes are RNA-protein complexes

composed of two subunits that join and

attach to messenger RNA.

– site of protein synthesis

– assembled in nucleoli

Endomembrane System

• Compartmentalizes cell, channeling passage of molecules through cell’s interior.

– Endoplasmic reticulum• Rough ER - studded with ribosomes

• Smooth ER - few ribosomes

Rough ER

• Rough ER is especially abundant in cells that secrete

proteins.

– As a polypeptide is synthesized on a ribosome

attached to rough ER, it is threaded into the

cisternal space through a pore formed by a protein

complex in the ER membrane.

– As it enters the cisternal space, the new protein

folds into its native conformation.

– Most secretory polypeptides are glycoproteins,

proteins to which a carbohydrate is attached.

– Secretory proteins are packaged in transport

vesicles that carry them to their next stage.

Smooth ER

• The smooth ER is rich in enzymes and plays a role in a variety of

metabolic processes.

The Golgi bodies/ dictyosomes

• The Golgi body is the shipping and receiving center for cell products.

– Many transport vesicles from the ER travel to the Golgi apparatusfor modification of their contents.

– The Golgi is a center of manufacturing, warehousing, sorting, and shipping.

– The Golgi apparatus consists of flattened membranous sacs—cisternae—looking like a stack of pita bread.

– The Golgi sorts and packages materials into transport vesicles.

Functions Of The Golgi Bodies

TEM of Golgi apparatus

cis face

(“receiving” side of

Golgi apparatus)

Vesicles move

from ER to GolgiVesicles also

transport certain

proteins back to ER

Vesicles coalesce to

form new cis Golgi cisternae

Cisternal

maturation:

Golgi cisternae

move in a cis-

to-trans

direction

Vesicles form and

leave Golgi, carrying

specific proteins to

other locations or to

the plasma mem-

brane for secretionVesicles transport specific

proteins backward to newer

Golgi cisternae

Cisternae

trans face

(“shipping” side of

Golgi apparatus)

0.1 0 µm16

5

2

3

4

Golgi

apparatus

Lysosomes and Peroxisomes

• Lysosomes – vesicle containing hydrolytic or digestive enzymes that break down food/foreign particles

• Peroxisomes - contain enzymes that catalyze the removal of electrons and associated hydrogen atoms

Lysosome

• Rounded or iregularly-shaped organelles bounded by a single membrane

• Said to play important role in the destruction of worn-out or defective parts of the cell

• Probably belong to a group of structures like microbodies

(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome contains

active hydrolytic

enzymes

Food vacuole

fuses with

lysosome

Hydrolytic

enzymes digest

food particles

Digestion

Food vacuole

Plasma membrane

Lysosome

Digestive

enzymes

Lysosome

Nucleus

Plastids

• Rounded, oval or irregularly-shaped

protoplasmic bodies of three main

parts:

• Leucoplast

• Chroloplast

• Chromoplast

Leucoplast

• Colorless plastids

• Some involved in food storage

• If associated with the starch storage, they

are referred to as amyloplasts

• if associated with oil storage= elaioplasts

• With protein storage = aleurone-plasts

Potato (Solanum tuberosum)starch grains

amyloplasts

Chloroplasts

• A chloroplast is bounded by two membranes enclosing a fluid-filled stroma that contains enzymes.

• Membranes inside the stroma are organized into thylakoids that house chlorophyll.

• Chlorophyll absorbs solar energy and carbohydrates are made in the stroma.

Digman cells (Hydrilla verticillata)

Chromoplast

• Colored plastids other than green

• The main pigments are the carotenoids

Bell pepper

• the thickest fibers, are hollow rods about 25

microns in diameter.

– are constructed of the globular protein,

tubulin, and they grow or shrink as more

tubulin molecules are added or removed.

• They move chromosomes

during cell division. Fig. 7.21b

Microtubules

• long, hollow cylinders and made-up

of protein tubulin

• outer diameter of 25nm

• much more rigid

• long and straight and typically have

one end attached to a single

microtubule-organizing center

(MTOC) called a centrosome

The Nucleus

• Repository for genetic material• Chromatin: DNA and proteins• Nucleolus: Chromatin and ribosomal

subunits - region of intensive ribosomal RNA synthesis. darkly staining rounded bodies rich in ribosomal RNA, the type of RNA used in the formation of ribosomes

• Nuclear envelope: Surface of nucleus bound by two phospholipid bilayer membranes -Double membrane with pores

• Nucleoplasm: semifluid medium inside the nucleus

Nucleus

Nucleus

– Nickname: “The Control Center”

– Directs cell activities

• Separated from cytoplasm by nuclear

membrane

• Contains genetic material - DNA

The Nucleus And The Nuclear Envelope

Chromosomes

• DNA of eukaryotes is divided into linear chromosomes.

– Exist as strands of chromatin, except during cell division

– Histones associated packaging proteins

3. The Inclusions

• Refer to the nonprotoplasmic structures found

within the protoplast

• Among these are the VACUOLES which are

fluid-filled structures

• The fluid vacuole commonly referred to as the

cell sap

• Contains various dissolved substances such

as anthocyanins (water-soluble pigments) and

various metabolites (e.g. Sugars, inorganic

salts, organic acids, alkaloids)

Cont...

• Other inclusions are waste products which may be in a form of crystals

• The crystals may be contained in vacoules and in the cytoplasm

• Plant cell may start with many but small vacuoles

• Upon maturity, the small vacuoles may coalesce to form bigger but fewer vacuoles

• Eventually, a plant cell may have but a SINGLE LARGE CENTRAL VACUOLE

• Some biologist would consider vacuole as an oganelle. As such the membrane-bounded structure containing the cell sap.

Crystals in Plant Cells

• Many plant cells contain crystals

which are a product of metabolism

• There are many forms

• Most common are composed of

calcium oxide and calcium carbonate

Calcium oxalate crystals

• Generally found in the vacuoles and are as follows:

1. Raphides – needle like crystals which occur singly

or in groups or bundles as in gabi and other succulent plants

2. Prismatic – prism-like or pyramid-like crystals found

in leaves of begonia and bangka-bangkaan

(Rhoeo discolor)

3. Rosette – aggregate of crystals which has

flower-like apperance in santan (Ixora sp.) and

stem of Kutsarita plant

Crystalline bodies in plant cells

Hypoestes phyllostachya, the polka dot plant

Crystalline bodies in plant cells

onion scales

Crystalline bodies in plant cells

Calcium carbonate crystals

• Cystotith- grape-like as seen in a hypodermal cell of the

leaf of Indian rubber tree or ampalaya-like plant.

BASIC TYPES OF CELLS

• Despite the diversity of types of stems that have

originated by natural selection, all share a basic, rather

simple organization.

• The same is true for leaves and roots.

• Although we might suspect that numerous types of cells

are present within a plant, actually the various kinds of

plant cells are customarily grouped into three classes

based on the nature of their walls:

Parenchyma, Collenchyma, and Sclerenchyma.

1. PARENCHYMA

• Parenchyma cells have only primary walls that remain thin.

• Parenchyma tissue is a mass of parenchyma cells. This is the most common type of cell and tissue, constituting all soft parts of a plant.

• Parenchyma cells are active metabolically and usually remain alive once they mature.

• Numerous subtypes are specialized for particular tasks:

Parenchyma cells of Geranium spp.; their walls (green) are thin, and their vacuoles are large and full of watery contents that did not stain. Nuclei were present in all cells,

Subtypes of Parenchyma Cells

A. Chlorenchyma cells

B. Glandular cells

C.Transfer cells

D. (Aerenchyma cells)

A. Chlorenchyma cells

• Chlorenchyma cells are parenchyma cells involved in

photosynthesis

• they have an abundance of chloroplasts, and the thinness

of the wall is advantageous for allowing light and carbon

dioxide to pass through to the chloroplasts.

• Other types of pigmented cells, as in flower petals and

fruits, also must be parenchyma cells with thin walls that

permit the pigments in the protoplasm to be seen.

Chlorenchyma cells from a leaf of privet- Ligustrumvulgare L.

-Intercellular spaces, permit CO2 rapid diffusion in the leaf

http://academic.kellogg.edu/herbrandsonc/bio111/tissue.htm

B. Glandular cells

• Glandular cells that secrete nectar, fragrances, mucilage,

resins, and oils are also parenchyma cells; they typically

contain few chloroplasts but have elevated amounts of

dictyosomes and endoplasmic reticulum.

• They must transport large quantities of sugar and

minerals into themselves, transform them metabolically,

then transport the product out.

Mauseth, J.

C. Transfer cells

• Transfer cells are parenchyma cells that mediate the short-

distance transport of material by means of a large, extensive

plasma membrane capable of holding numerous molecular

pumps.

• Unlike animal cells, plant cells cannot form folds or

projections of their plasma membranes; instead, transfer

cells increase their surface area by having extensive knobs,

ridges, and other ingrowths on the inner surface of their walls

• Because the plasma membrane follows the contour of all

these, it is extensive and capable of large-scale molecular

pumping.

Transfer cells in the salt gland of

Frankenia grandifolia.

The wall ingrowths increase the

surface area of the cell membrane,

providing

more room for salt-pumping proteins in

the membrane.

W. W. Thomson

and R. Balsamo, University of

California, Riverside

D. (Aerenchyma cells)

• Parenchyma tissue with extensive connected air spaces.

• refers to spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and the root.

• The channels of air-filled cavities provide a low-resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant above the water and the submerged tissues.

• Aerenchyma is widespread in aquatic and wetland plants which must grow in hypoxic soils

Stem cross-section of Peperomia. The inner part of the stem is mostly parenchyma cells.

• Some parenchyma cells function by dying at maturity.

• Structures such as stamens and some fruits must open and release pollen or seeds; the opening may be formed by parenchyma cells that die and break down or are torn apart.

• Large spaces may be necessary inside the plant body; some of these are formed when the middle lamella decomposes and cells are released from their neighbors.

• In other cases, the space is formed by the degeneration of parenchyma cells.

• In a few species, such as milkweeds, as parenchyma cells die, their protoplasm is converted metabolically into mucilage or a milky latex.

• Parenchyma tissue that conducts nutrients over long distances

is phloem

• Parenchyma cells are relatively inexpensive to build because

little glucose is expended in constructing the wall's cellulose

and hemicellulose.

• Each molecule incorporated into a wall polymer cannot be

used for other purposes such as the generation of ATP or the

synthesis of proteins.

• Consequently, it is disadvantageous to use a cell with thick

walls any time one with thin walls would be just as functional.

2. COLLENCHYMA CELLS

• Collenchyma cells have a primary wall that

remains thin in some areas but becomes thickened

in other areas, most often in the corners.

• The nature of this wall is important in understanding

why it exists and how it functions in the plant.

• Like clay, the wall of collenchyma exhibits plasticity,

the ability to be deformed by pressure or tension and

to retain the new shape even if the pressure or tension

ceases.

• Collenchyma is present in elongating shoot tips that must be long and flexible, such as those of vining plants like grapes, as a layer just under the epidermis or as bands located next to vascular bundles, making the tips stronger and more resistant to breaking.

• But the tips are still capable of elongating because collenchyma can be stretched.

• In species whose shoot tips are composed only of weak parenchyma, the tips are flexible and delicate and often can be damaged by wind; the elongating portion must be very short or it simply buckles under its own weight.

Stem cross-section of Peperomia. Masses of collenchyma cells often occur in the outer parts of stems and leaf stalks. The collenchyma forms a band about 8 to 12 cells thick.

• It is important to think about the method by which collenchyma provides support.

• If a vine or other collenchyma-rich tissue is cut off from its water supply, it wilts and droops; the collenchyma is unable to hold up the stem.

• Parenchyma cells are needed in the inner tissues for support. Collenchyma and turgid parenchyma work together like air pressure and a tire: The tire or inner tube is extremely strong but is useless for support without air pressure.

• Similarly, air pressure is useless unless it is confined by a container.

• In stems, the tendency for parenchyma to expand is counterbalanced by the resistance of the collenchyma, and the stem becomes rigid.

The shoot tips

of long vines

need the plastic

support of

collenchyma

while their

stems are

elongating

• Because the walls of collenchyma cells are thick, they

require more glucose for their production.

• Collenchyma is usually produced only in shoot tips and

young petioles, where the need for extra strength justifies

the metabolic cost.

• Subterranean shoots and roots do not need collenchyma

because soil provides support, but the aerial roots of

epiphytes such as orchids and philodendrons have a

thick layer of collenchyma

3. SCLERENCHYMA CELLS

• The third basic type of cell and tissue, sclerenchyma,

has both a primary wall and a thick secondary wall that

is almost always lignified.

• These walls have the property of elasticity: They can be

deformed, but they snap back to their original size and

shape when the pressure or tension is released.

• Sclerenchyma cells develop mainly in mature organs that

have stopped growing and have achieved their proper

size and shape.

A stem of bamboo was treated with a mixture of nitric acid and chromic acid to

dissolve the middle lamellas and allow the cells to separate from each other.

These are sclereids; they are more or less cuboidal, definitely not long like fibers.

These have remained alive at maturity, and nuclei & cytoplasm are visible in

several

astrosclereids (star-shaped sclereids), shines brightly because its cellulose molecules

are packed in a tight, crystalline form, giving the wall extra strength .

• Deforming forces such as wind, animals, or snow would probably be detrimental.

• If mature organs had collenchyma for support, they would be reshaped constantly by storms or animals, which of course would not be optimal.

• For example, while growing and elongating, a young leaf must be supported by collenchyma if it is to continue to grow.

• But once it has achieved its mature size and shape, some cells of the leaf can mature into sclerenchyma and provide elastic support that maintains the leaf's shape.

• Unlike collenchyma, sclerenchyma supports the plant by its strength alone; if sclerenchyma-rich stems are allowed to wilt, they remain upright and do not droop.

• Parenchyma and collenchyma cells can absorb water so powerfully that they swell and stretch the wall, thereby growing; sclerenchyma cell walls are strong enough to prevent the protoplast from expanding.

• The rigidity of sclerenchyma makes it unusable for growing

• shoot tips because it would prevent further shoot elongation.

• Sclerenchyma cells are of two types—conducting sclerenchyma and mechanical sclerenchyma.

• The mechanical sclerenchyma is subdivided into long fibers and short sclereids , both of which have thick secondary walls.

• Because fibers are long, they are flexible and are most often found in areas where strength and elasticity are important.

• The wood of most flowering plants contains abundant fibers, and their strength supports the tree while their elasticity allows the trunk and branches to sway in the wind without breaking (usually) or becoming permanently bent .

• The fiber-rich bark is important not in holding up the tree but in resisting insects, fungi, and other pests.

• Sclereids are short and more or less isodiametric(cuboidal).

• Because sclereids have strong walls oriented in all three dimensions, sclerenchyma tissue composed of sclereidsis brittle and inflexible.

Mauseth, J.

SUMMARY

Parenchyma This tissue is composed of large,

thin walled cells having a single large vacuole.

This tissue is found in the soft parts of the plant like the cortex(outer region) and pith(inner region) of the roots and stems.

The cells present in these tissues are living.

These cells store food for the plant, and thus they also provide temporary support for the plants.

When parenchymatous cells are present in leaves, they sometimes contain chlorophyll, and thus are green in colour. This tissue is then referred to as chlorenchyma.

Potatoes are made up of mainly Parenchyma tissue cells.

Collenchyma This tissue is made up of

cells from the parenchyma tissue which become elongated and have thick cell walls at the corners. This diagram illustrates the difference between parenchyma and collenchyma cells. The collenchyma cells are elongated and thick at the corners.

This tissue is found mainly in the leaf stalks and below the epidermis of stems.

It gives support for the parts of a plant like leaves, stems, branches.

Sclerenchyma

Sclerenchyma is made up of thin, narrow and long cells having very thick cell walls due to deposition of lignin.

These cells are thin because they are all dead cells, having no functions to perform and so no organelles inside the cell require space.

They are thick at the cell walls and thus these cells provide rigid support for the plant as they are hard and supportive. That is why the tissue is named "Sclerenchyma", as scleros means hard.

Sclerenchyma tissue is present in stems of plants and the veins of leaves in plants. It provides strength to plant parts.